Just read a blues post and it said:
By reading this, we can confirm that we can get 2 per week guaranteed without any luck. I just recently started the quest myself, finished secrets right before i started and had only one LFR part i could do for loot, did it and got 1 runestone for the week. Seems pretty neat to me because it was 12 weeks without any luck.It's only 1 attempt per boss per week across all difficulties.
If you already completed Siege of Orgrimmar wings 1 and 2 for Secrets of the Empire they won't drop a Titan Runestone. If you haven't completed Throne of Thunder wings 3 and 4 though, you can run that with Lei Shen being a 100% guaranteed drop..
As a side note, only the first 8 bosses in SoO have a chance (Sha of Pride is 100% guaranteed just like Lei Shen) to drop items for Wrathion's legendary quest line.